The worldwide market for Chemical Vapor Deposition (CVD) equipment and services is projected to grow from $3.5 billion (per year) in 1995 to $5.4 billion by the year 2000. The two major industrial sectors are: (a) microelectronics which currently accounts for approximately 80% of the market, and (b) surface coatings applications (surface hardness enhancement, corrosion inhibition, and medical) which accounts for the remaining 20% of the market. The microelectronics industry is concerned with stringent purity requirements, while the surface coatings industry is primarily concerned with surface hardening (for machine tooling) and surface coating (for corrosion inhibition and medical applications). While the microelectronics industry can incur a significant expense to produce a high-purity film with exacting electrical performance, many surface enhancement applications cannot tolerate the high production costs typical of the present-day CVD techniques. Additional challenges that both industries face are the expense of disposing of CVD byproducts and the high surface temperature requirement for many conventional CVD processes (900.degree. C. to 1200.degree. C. for thermal CVD of titanium). This latter point is of significance because many substrates may not be capable of accepting high deposition temperatures. For example, mild steel (less than 0.25% C) undergoes a phase transition at elevated temperatures e.g., 723.degree. C., such a change affecting the steel's properties.
Various techniques have been proposed to form thin films using CVD. One technique uses halides such as titanium halide with silanes and ammonia as disclosed in U.S. Pat. No. 5,595,784 to Kaim et al.
In spite of the prior art techniques known to date, a need has developed to provide a CVD process for making thin films on substrates that is low cost, is environmentally benign, uses low temperatures and provides reasonable purity levels. The present invention solves this need by developing a CVD technique that uses a metal-containing halide vapor and an alkali metal vapor to produce a thin film structure.
The generation of bulk titanium by reacting titanium halide vapor and sodium vapor is known. This chemistry has also been demonstrated to be useful for a variety of applications including the synthesis of titanium nanoparticles and TiB.sub.2 nanoparticles, for the formation of an iron/salt magnetic nanocomposites, for the study of phase segregation in binary SiO.sub.2 /TiO.sub.2 and SiO.sub.2 /Fe.sub.2 O.sub.3 nanoparticles, for the synthesis of high purity Si (near photovoltaic grade, and for the remediation of chlorinated fluorocarbons (CFC's))
However, the use of alkali metals, e.g., sodium and potassium, has not been suggested for use in industrial chemical vapor deposition applications primarily because their reductive power is so great that it tends to cause premature and detrimental gas-phase particle precipitation. Furthermore, the deposition of salts is problematic in that high temperatures are required to volatilize the deposited salt.